Dyeing is as old as the textile industry and this antedates the written documents of human history. The natural desire of humans to clothe themselves goes hand in hand with the desire for beautiful, colorful clothing. There is historical evidence that indigo a vat dye, was used on the island of Crete as far back as 1600 B.C. for coloring clothing. Indigo remains the most widely recognized vat dye for dyeing clothing although most vat dyes have been in commercial use for at least 50 years.
Indigo was historically made from sea shells. Today the principle source of indigo is from plants which contain a compound called Indican. Indican is water soluble due to its glycoside linkage at the third position. To produce indigo from the plant material, the following procedure is generally used. The Indican is placed in an aqueous solution, the glycoside linkages are broken and 3-oxindole is released. The Indican/aqueous solution is then saturated with air which provides oxygen so that the 3-oxindole dimerizes to form the dye indigo. The dye, indigo, that is formed, is blue in color and is water insoluble. Likewise, the dye is practically insoluble in alcohol, ether or dilute acids. To use indigo to color textiles, it is necessary to reduce the indigo dye to its leuco form using a suitable reducing agent. This reduction usually occurs in an alkaline material, for example sodium hydroxide. Once the ,, leuco form of the dye is achieved the textile is immersed in the leuco solution and then exposed to air. These two steps, immersion and exposure may be repeated to obtain the desired shade of blue. Although there has been a widespread use of dyes for coloring fabrics, there is still a need to improve the existing dyeing processes and particularly a need to improve the step of reducing the dye to its soluble leuco form.
Typically, sodium dithionite has been employed to reduce the dyes to their leuco form, as is shown U.S. Pat. No. 3,798,172. Sodium dithionite decomposes rapidly in the presence or absence of air; therefore, large, excess quantities of this compound are used to reduce all of the dye and to maintain the dye in is soluble leuco form. Regrettably, the use of sodium dithionite raises ecological concerns.
Other reducing systems have been recommended for use by the textile industry. These include a highly alkaline solution containing five percent sodium borohydride and a powdered initiator of sodium formaldehyde sulfoxylate (Rongalite C) with five percent sodium nickel cyanide. There are some environmental concerns relating to the nickel and cyanide discharges from this dyeing process. Another reducing agent that has received some attention is thiourea dioxide in an alkaline solution. However, thiourea dioxide is a strong reducing agent, and therefore tends to over reduce the vat dye. When reducing indigo overreduction is readily apparent by the smell of fecal material which is present when indole (the overreduction product) is in the reaction.
The reduction process is sensitive to slight changes in pH, temperature, and the amount and purity of the material being reduced. If the pH is too low, the dye may precipitate in its acid form. If sodium dithionite is used in too low a concentration, the dye may oxidize prematurely, resulting in a loss of the dye. Therefore, despite the fact that vat dyes have been known for years, the quest for an optimal, inexpensive reducing process which does not have environmental concerns attached to its use or which does not result in overreduction is ongoing.
Because the industry has not found an acceptable process for forming the leuco form of vat dyes the industry has turned to the formation of indigosols, the sulfuric esters of the leuco form of vat dyes. The reduction and esterification of the vat dye produces the sulfuric ester instead of the leuco form of the vat dye. Pyridine with iron and chlorosulfuric acid are agents employed to form the sulfuric ester having the general structure of ##STR1## The industry would prefer to form the leuco form of the vat dye instead of forming the ester to avoid adding expense to the dyed fabric.
The present invention provides an optimal reducing process for formation of the leuco form of the vat dye. Thus eliminating the need to form sulfuric esters.